Laminated electrochemical cells are typically arranged in a stack configuration and interconnected to form larger power producing devices, such as modules or batteries. A grouping of electrochemical cells may be selectively interconnected in a parallel and/or series relationship to achieve a desired voltage and current rating.
It has been determined that the performance and service-life of such modules or batteries are significantly improved by maintaining the layers of the stacked electrochemical cells in a state of compression. Improved cell performance may be realized by maintaining pressure on the two larger opposing surfaces of the cells during cell cycling. The thermal conduction characteristics of a stack of electrochemical cells are significantly improved when forced contact between adjacent cells is maintained. It is considered desirable that the compressive forces be distributed uniformly over the surface of application.
One factor that complicates the effective thermal and electrical conduction for thin-film electrochemical cells in a stack configuration is the cyclical changes in cell volume that occur during charge and discharge cycles. The volume of an electrochemical cell varies during charge and discharge cycling due to the migration of ions, for example lithium ions, into and out of the lattice structure of the cathode material. This migration causes a corresponding increase and decrease in total cell volume in the order of as much as ten percent during charging and discharging, respectively. The volume of the cells also fluctuates with temperature variation such that thermal dilatation and contraction may represent as much as a five percent increase and decrease, respectively, in total cell volume. In modules or batteries comprising numerous thin-film electrochemical cells in a stack configuration, the volume change is compounded such that the overall volume change is significant and must be accommodated.
In order to accommodate these compounded variations in electrochemical cell volume resulting from charge and discharge cycling of a grouping of cells, a pressure producing apparatus within the walls of the containment vessel of the battery is employed to maintain the cells in a continuous state of compression. An active pressure generating mechanism, such as a foam element or a spring-type element adjacent to the walls of the containment vessel is used to apply an evenly distributed pressure onto the outer surfaces of the outer cells of the cell stack during charge/discharge cycling. For large battery applications, the active pressure generating mechanism is typically comprised of a plurality of metal springs applying pressure against a metal plate which can generate the necessary compressive force, and may include spring inserts located between adjacent cells within the cell stack to enhance distribution of compressive forces within the cell stack.
In the manufacturing of stacked electrochemical cells, the electrochemical cells are stacked between a pair of pressure producing apparatus, the assembly is introduced into an hydraulic press where it is compressed to the target pressure and the assembly is mechanically tied together with a series of straps to maintain it at the target pressure. In the strapping operation, each strap is wrapped around the assembly of stacked electrochemical cells and pressure producing apparatus and spot welded to lock it in position. The wrapping and welding operation is time-consuming and requires an elaborate apparatus comprising an hydraulic press, a wrapping device and a welding head.
Thus, there is a need in the industry for a simple and efficient device for maintaining an assembly of stacked electrochemical cells under a state of compression.